When you have finished cutting the text elements, adjust all the parts of the poster & step back to take a good look at it. If it appears crooked, it probably is. Trust your eyes and re-trim the offending piece.
Before you trim those elements which are more difficult to replace (photographs, etc.), lay the poster out again and have someone else view it with you. NOW is the time to look for errors in the text and correct them. If you do make any changes in the text, save those changes!
The next step is to actually Assemble all the bits and pieces. Or, you can RETURN to the Main Menu page.